The present invention relates generally to clutches, especially for automobile vehicles, and more specifically to their control systems.
As is well known, a clutch usually comprises a reaction plate which is intended to be keyed in rotation with a driving shaft, (in practice this is the output shaft, or crankshaft, of the engine of an automobile vehicle), a friction disc mounted to be axially movable relative to the reaction plate and keyed in rotation with a driven shaft, (in practice usually the input shaft of the gearbox) a pressure plate which is also mounted to be axially movable relative to the reaction plate and which is adapted to clamp the friction disc against the latter, a cover, which is axially fastened to and rotatable with the reaction plate, elastic clamping means which bear on the cover and which are adapted to urge the pressure plate towards the reaction plate in order to clamp the friction disc and thus engage the clutch, and clutch release means operated by a control system and adapted to release the friction disc to disengage the clutch.
As is also well known, the clutch control system usually incorporates a clutch release bearing which, in response to control means, is adapted to act axially in a positive manner in one direction on the clutch release means.
The term "positive" means an action which normally does not involve any elastic means, any displacement of the clutch release bearing in the direction concerned resulting in an equally large displacement of the clutch release means on which it acts.
If, in order to move into the disengaged configuration of the clutch, the clutch release bearing acts in the axial direction corresponding to an approach of the clutch release bearing towards the reaction plate, the clutch is of the kind known as a "pushed" clutch, while if for this movement into the disengaged position, the clutch release bearing moves axially further away from the reaction plate, it is known as a "pulled" clutch.
Whatever the arrangement, when a clutch is in the engaged configuration, with its friction disc clamped between the pressure plate and the reaction plate, there is usually, whilst the torque to be transmitted remains below a set nominal value, integration in rotation, without slipping, of the pressure plate and the reaction place, on the one hand, and the friction disc on the other hand.
However, during a transient phase, and this is the case particularly after a reversal of the torque as may be produced when, with the vehicle moving, the driver rapidly releases its accelerator and when the operation of the assembly consequently just as rapidly changes from a "pulling" mode in which the engine drives the vehicle, to a "retro" mode in which the vehicle tends to drive the engine, excess torques may be developed, that is to say torques with a value greater than the nominal value mentioned above.
In such a case, slipping normally occurs between the friction disc and the pressure and reaction plates which enclose it.
For various reasons and particularly to minimize the resultant wear of the friction disc, it is usual under these conditions to overdimension the clutch by a safety factor of e.g. about 1.3, such that it will be able to tolerate without slipping, a change of torque of a value higher than a predetermined nominal value selected to correspond to the maximum engine torque.
This arrangement which has been and still is satisfactory, at least for certain applications, has a double disadvantage.
First of all, it necessitates, as a matter of principle, overdimensioning of the clutch, to the inevitable detriment of its diametric bulk, its weight and its cost.
Moreover, whilst near the nominal value of the torque to be transmitted oscillations with relatively low amplitude are produced, corresponding to the unavoidable vibrations which may arise all along the kinematic chain, these oscillations do not induce slipping if they remain within the limits of the safety factor applied, and consequently the corresponding vibrations are integrally transmitted by the clutch to the detriment of the user's comfort.
The present invention has a construction which permits in a very simple manner these disadvantages to be minimized or even eliminated.